This invention relates to a crankshaft casting pattern and method and more particularly, to a crankshaft casting pattern and method that produces crankshafts which have an improved rotational balance capability and which can be measured with improved accuracy.
Engine crankshafts are typically formed by use of a casting procedure or methodology. One type of casting process, commonly referred to as a xe2x80x9cgreen-sand foundryxe2x80x9d process, utilizes an upper pattern shape of the desired crankshaft called a xe2x80x9ccope patternxe2x80x9d and a lower pattern shape of the desired crankshaft called a xe2x80x9cdrag patternxe2x80x9d. During the green sand foundry process, the pattern shape is impacted into a xe2x80x9cgreen-sandxe2x80x9d flask, thereby leaving an imprint in the sand corresponding to the desired casting shape. The upper cope pattern forms the upper half of the part and the lower drag pattern forms the lower half of the part.
When crankshafts are produced through the green-sand process, several casting shapes or pattern inserts are mounted on a plate to produce multiple parts in the molding process (e.g., two, four, six, eight or more parts may be made simultaneously). Making multiple parts in one mold improves production efficiency. The desired shape of the finished casting is a function of the shape of the pattern insert, the quality properties of the mold sand, the alignment of the upper mold half (i.e. the cope portion) to lower mold half (i.e. the drag portion), and the shrink rate of the molten metal as it cools.
The metal used to form the crankshafts is typically a high nodularity (or ductile) type iron. The nature of making green-sand high nodularity cast-iron crankshafts requires that the mold sand properties be dimensionally consistent across the entire mold so that all cast parts have the same desired dimensional consistency. Also, the pattern inserts must be dimensionally consistent from insert to insert so that the cast parts have the same desired dimensional consistency. The alignment accuracy of setting the cope mold flask onto the drag mold flask has a great influence on the dimensional consistency of the cast part.
One of the most influential characteristics of the green-sand high nodular cast-iron foundry process is the solidification nature of the molten iron, as nodular iron has a great tendency to swell during solidification. This swell tendency causes the iron to push outward on the mold sand resulting in mold deformation. If mold deformation during solidification is constant across the entire mold sand shape, then all crankshaft castings will have the same dimensional consistency. However, if mold sand deformation during solidification is not constant across the entire mold, then the dimensions of the castings will have slight variations. These slight variations cause the produced crankshafts to have varying shapes from insert to insert, which leads to changes in the mass properties of the crankshafts which ultimately causes variations in the production mass balancing process.
Pattern inserts that are positioned at the outer edges of the flask have mold sand properties different from the pattern inserts that are positioned at the inside of the flask. As a result, mold sand deformation varies across the mold, allowing for cast parts at the inside of the mold to have slight dimensional differences from cast parts at the outer edge of the mold. This is due to density differences in mold sand. The varying densities cause the mold sand to react differently to iron solidification at different locations within the mold. Because the various crankshaft castings produced by the mold have slight dimensional variations due to their respective positions in the mold, the crankshafts will have slightly different mass properties which influence the ability to balance the crankshaft in a repeatable process.
Particularly, because the crankshaft castings vary slightly in dimensional properties, the mass balance properties will have a similar variation in the machining/balancing process of manufacturing. With variations in the casting process, it becomes necessary to sort castings by insert or pattern number and then machine and balance as a given insert number. This requires sorting (e.g., batching) castings by their respective insert or pattern number and then processing by insert number through the machining and balancing process. This sorting and running by insert number requires that the machine centering operation be adjusted accordingly to insure proper mass balance capability of the finished crankshaft. Thus, every crankshaft insert may require a different machine centering set-up to process, which substantially increases production cost and reduces efficiency.
The unique nature of the machining and balancing process causes these problems. Particularly, when a crankshaft rough casting is delivered to the machining station, the first operation of machining is to add the rotational center drill operation at each end of the crankshaft. After the centering operation, all machining features are sequentially completed from these centers which is the main machining feature datum. This machined centering datum is required to be on the near exact center of the rough casting in order to ensure proper mass balancing at the last machining operation. Thus to ensure that the machined part is balanceable, the casting mass center must be within the mass center of the machining operation. Any slight differences are compensated for by the balancing operation which drills away material at the end counter-weights to bring the part within mass-balance specification. However, any variation in casting features may result in a crankshaft that is not balanceable due to the mass properties which are out of the range of balancing equipment capabilities.
To insure that the rough casting datums are within the machined center datums, there requires a critical interface between casting datums and machining datums. This is accomplished by using the end main bearing journals which are clamped into centering jaws of the machine which adds the crankshaft centers. FIG. 6 illustrates the machine clamping jaw configuration at the crankshaft centering operation.
FIGS. 6a-6c illustrate the effect of size variation on the location of the journal when clamped by the centering members or clamps 90, as well as an incoming casting size variation which results from pattern separation at the parting line of the mold. Mismatch of the mold causes an offset in the part which can effect the clamping jaws 90 which locate the machine centering datum or centerline. Journal roundness can also effect the centering members which can influence the centering operation of the crankshaft.
Thus, in order to have a correctly centered machining operation, it is essential to have a casting shape which is correct in diameter roundness, parting-line separation and mold mismatch to minimize dimensional variation.
There is therefore a need for a crankshaft casting pattern and methodology which produces rough crankshaft castings which are substantially similar in shape and size, thereby allowing all of the castings to be processed with one machine centering operation to ensure consistent mass balance with no variation.
It is a first object of the invention to provide a crankshaft casting pattern and method which overcomes some or all of the previously delineated drawbacks of prior crankshaft casting patterns, systems and methods.
It is a second object of the invention to provide a crankshaft casting pattern having a modifiable pattern shape which allows rough crankshaft castings to be altered, thereby improving their rotational balance capabilities.
It is a third object of the invention to provide a crankshaft casting pattern having a selectively modifiable pattern shape which produces crankshafts which require a reduced amount of drilling during balancing.
It is a fourth object of the invention to provide a crankshaft casting pattern and method which produces crankshafts which are substantially similar in shape and size throughout a mold, thereby substantially eliminating and/or reducing the need to sort castings prior to balancing.
It is a fifth object of the invention to provide a crankshaft casting pattern and method which allows critical dimensional features of a produced crankshaft to be accurately measured.
It is a sixth object of the invention to provide a crankshaft casting pattern which is selectively adjustable to compensate for pattern wear over time.
According to a first aspect of the present invention, a casting pattern is provided and includes at least one selectively adjustable portion, which allows said casting pattern to be selectively modified in shape.
According to a second aspect of the present invention, a method is provided for casting components. The method includes the steps of providing a plate having a plurality of adjustable pattern inserts; forming a plurality of molds by use of the plurality of pattern inserts; making a plurality of sample castings by use of the plurality of molds; dimensionally analyzing the plurality of sample castings; and adjusting the plurality of pattern inserts based upon the dimensional analysis, effective to ensure that all castings that are made from molds formed by the plate have substantially similar balance characteristics.
These and other objects, aspects, features, and advantages of the present invention will become apparent from a consideration of the following specification and the attached drawings.